The present invention relates to telecommunication systems, and specifically to telephony via a combination of circuit switched public telephone systems and packet based digital systems.
Telephony, which means “remote sound transmission” has been available for about a century. Public companies, usually regulated and taxed by government agencies, provide wiring to telephone customers, who lease or own a Telephone, and provide switching equipment allowing any pair of Telephones to be connected together for communication. A Telephone includes a microphone and a small loudspeaker, plus a keypad or a rotary dial for specifying which second Telephone the customer wants to be connected to. The global network for this type of telephony is known as the Public Switched Telephone Network (“PSTN”). Lately, PSTN has been expanded to carry facsimile messages and other digital information, and transmission may in part be via digital signals instead of analog, but the network is still circuit switched, which is characterized by continuous point to point communication between end users.
Cellular telephony is a variant of PSTN. Communication between a Cellular Telephone and the network is wireless, and may be analog or digital, but inside current networks circuit switching is used for connections.
The Internet is a Packet Based Digital Network (“PBDN”), which offers high capacity and low cost transmission of digital information. In the mid 1990's, enthusiasts started making voice connections between computers connected to the Internet, using microphones and loudspeakers connected to the computers, and software to make the computers converting a voice stream into digital packets transmitted over the Internet and back into a voice stream. This was the first form of what is called Internet Protocol telephony (“IP telephony”). Software for telephony via computers on the Internet is readily available today. This type of voice connection can, however, only be used between computers that are connected to the Internet before the voice connection is set up.
Another form of IP telephony is used for toll bypass, i.e. to reduce the cost of long distance calls. There are numerous providers offering reduced long distance rates by routing the long distance portion of a call over the Internet. Network Gateways are used as intermediaries between a PSTN and a PBDN at both the calling end and the receiving end of the connection. A customer uses an ordinary Telephone, but dials via a suitably located Network Gateway, which packetizes the voice signal and transmits the voice data packets via the Internet to another Network Gateway close to the called party. There the packetized voice data are converted back to voice signals for the PSTN. This type of IP telephony requires a rather cumbersome dialing sequence, and the voice quality may be inferior to circuit switched analog telephony, but the cost is lower than for ordinary long distance calls.
U.S. Pat. No. 5,751,706 issued to Land et al. gives a comprehensive description of the present state of the art of telephony gateways between PBDN and PSTN.
Standards for IP telephony, such as H.323 from the International Telecommunication Union (“ITU”) and Session Initialization Protocol (“SIP”) from the Internet Engineering Task Force (“IETF”), are under development. Early versions have been defined and continued work is in progress. However, current IP telephony equipment, both end user equipment and gateways, currently show poor compatibility. Future improved standards and better compliance will hopefully remedy this problem.
The lack of compatibility, and also limited cooperation between IP telephony service providers, have often forced providers to have their own Network Gateways at both ends of a call connection. Known systems for IP telephony are thus often limited to communications between initiators and receivers using Network Gateways operated by a specific IP telephony provider. A single Network Gateway can handle only a limited number of simultaneous telephone calls, so prior art IP telephony systems are limited by the investment in Network Gateways, both in its geographic coverage, and in its capacity in each geographic area.
The next step in the evolution of IP telephony will utilize new technologies for fixed high speed Internet connections, such as cable modem or Digital Subscriber Line (“DSL”) connections. These technologies will spur the development of special IP telephony apparatuses (“IP Telephones”), which will remain connected to the Internet at all times. A telephone connection between two compatible IP Telephones will be via packetized digital voice data directly over the Internet. Network Gateways are thus not required for direct connections, but calls between an IP Telephone and a Telephone connected to a PSTN will require a Network Gateway located close to the called Telephone. The Network Gateway and the IP Telephone must use the same IP telephony standard and be compatible. This known system for IP telephony thus suffers from the same limitations as any system based on Network Gateways when the IP Telephones are used for connections with ordinary Telephones connected to a PSTN.